How do I achieve this simple shape?
you should try to mark those edges and use the bevel modifier with the option set to bevel weight, instead of using way too many cuts like you did.
Dissolve all edges loops between the two cuts.
OR use a bevel on a sharp corner edge with a suitable amount of division, like in my drawing. Usually, 3 or 4 cuts are enough to achieve the effect you want, provided that you also have a sufficient amount of geometry (for example a subdivision modifier in the stack).

Hope it helps. Check my drawing on your example.